FireWire Cables and Connectors
FireWire devices can be powered or unpowered. FireWire allows devices to draw their power from their connection. Two power conductors in the cable can supply power (8 to 30 volts, 1.5 amps maximum) from the computer to an unpowered device. Two twisted pair sets carry the data in a FireWire 400 cable using a 6-pin configuration.
Some smaller FireWire-enabled devices use 4-pin connectors to save space, omitting the two pins used to supply power.
FireWire 800 cables use a 9-pin configuration. Six of those pins are the same as the six pins in the 1394a connector (shown above). Two of the added pins provide a "grounded shield" to protect the other wires from interference, and the third added pin does nothing at this time [ref].
Because FireWire 800 is backward-compatible with FireWire 400, there are a variety of adapters available to facilitate the combination of both standards on the same bus. There are also two types of FireWire 800 ports available: a "bilingual" port accomodates both FireWire standards, while a b-only port accepts only a FireWire 800 connector.